Lamps or lighting devices which comprise light emitting diodes (LEDs) may contain LEDs of several colors in order to either allow color adjustment or enable a higher color rendering index (CRI). For such devices there is a desire to provide light which is uniform in regards to color and intensity in the far field, e.g. at the surface, object or room which is being lit by such a device but also in the near field e.g. when an observer is looking at the device. However, in order to achieve an efficient CRI using several LEDs of different color it is common to use several reflectors or collimators which results in different colors being observed in the near field. A solution to this particular problem is to use a single collimator which collimates the light from several LEDs with additional mixing means. One known solution consists of a total internal reflection (TIR) collimator which has faceting on the exit aperture in order to diffuse the light, in some cases this is not enough and faceting is used on the internal reflection surfaces or even including diffusing elements within the body of the TIR collimator.
However, even in devices using a single collimator the mix of colors is still incomplete such that different colors are still visible when observing the near field e.g. looking into the collimator. Further, the faceting provides a wide beam from the collimator, which will not enable a narrow and mixed beam of light i.e. there will be a strong Etendue dilution.
U.S. Pat. No. 6,139,166 propose using a beam splitter to mix light from LEDs which emit light of different colors. However, color mixing of several sources requires more than one mixing stage and the proposed device uses a prismatic block comprising mirrors which may result in losses.
Hence, there is still a need for a lighting device which can provide a narrow beam while mixing light from several light sources in both the near field and far field.